class code7 {
    //找到排序数组0-n-1中缺失的数
    public int takeAttendance(int[] records) {
        int left =0,right = records.length-1;
        while(left<right) {
            int mid = left +(right-left)/2;
            if(records[mid]==mid) left = mid+1;
            else right = mid;
        }
        return records[left]==left?left+1:left;
    }
}